When President Obama recently authorized humanitarian aid and air strikes in northern Iraq, Seth Moulton, a war veteran and Democratic candidate for Congress, quickly put out word on where he stood.

Humanitarian assistance is fine — but the United States should not send ground troops back to Iraq. Moulton also opposes sending to Iraq military advisers, his very own role in the country a decade ago.

If, as Moulton is, you’re running for office in liberal Massachusetts, and especially if you are running against John Tierney, an incumbent who voted against invading Iraq in 2003, you walk a careful line on matters of war and peace. Saber-rattling is not a winning strategy, even in the state’s more conservative Sixth District.

Elsewhere in the country, Moulton’s combat credentials would be a blue-chip asset. Here in Massachusetts, respect for his veteran status runs up against disrespect for an unpopular war foisted on a country under false pretenses.

From the Tierney camp, it’s also cause for skepticism about Moulton’s true ideology, as if a military background automatically disqualifies a candidate as a liberal.

Asked during a recent visit to the Globe if he is more conservative than his opponent, Moulton insisted he is not. The difference, he said, is that he believes in “working with Republicans” and does not focus on “how evil the Republicans are.”

Moulton is used to ambivalence over his military record and what it represents. His mother, Lynn, told Boston Magazine in 2008 that when her son, then a senior at Harvard, said he planned to join the Marines, her first thought was, “There was no career choice he could have made that would have made me more unhappy, except if he had chosen a life of crime.”

His parents opposed the Vietnam War as college students in the late 1960s, and, as Lynn Moulton also told Boston Magazine, “I didn’t raise my son to be a Marine.”

She did not have the final word. At Harvard, Moulton was mentored by the late Peter J. Gomes, a theologian and professor of christian morals, who spoke to him about service to country. Their discussions led to Moulton’s decision to enlist in May 2001, right before his Harvard graduation and four months before the Sept. 11 terrorist attacks.

According to his campaign website, Moulton led an infantry platoon during the 2003 invasion and was in the first Marine company to enter Baghdad. He went on to serve four tours of duty and work as a special assistant to then-Lieutenant General David Petraeus. After his final tour, he returned to Harvard, where he earned an MPA from the John F. Kennedy School of Government and an MBA from Harvard Business School.

Tierney, 62, was first elected in 1996. Even with four Democrats challenging him, he appears in no danger of losing the primary. Yet, with Iraq back in the headlines, Moulton’s military record offers an interesting contrast to Tierney’s staunch, anti-war voting record.

The Tierney camp likes to point out that when Tierney voted against the Iraq invasion, his constituents favored the military campaign, and the newspapers in his district backed it up on their editorial pages.

But now, as public opinion on that war effort has shifted everywhere, Moulton is the one who has to distance himself from Iraq. He said he was proud to serve but calls the war a mistake.

“I saw a lot of friends and colleagues die for a war that should never have happened,” said Moulton, 35, in an interview. “The last thing I want to see is for it to happen again.”

Both he and Tierney would vote against war today, Moulton added, but, “We need more people in Congress who have on the ground experience and credibility to ask the tough questions.”

This job description for a platoon leader would guide him in Congress, said Moulton: “You are responsible for everything your platoon does or fails to do.”

A Tierney adviser who did not want to be named frames the war issue like this: “Moulton was brave enough to go and smart enough to change his mind. Tierney was smart enough to oppose the war from day one and brave enough to do so when it put him in personal political peril.”

BostonGlobe.com complimentary digital access has been provided to you, without a subscription, for free starting today and ending in 14 days. After the free trial period, your free BostonGlobe.com digital access will stop immediately unless you sign up for BostonGlobe.com digital subscription. Current print and digital subscribers are not eligible for the free trial.

Thanks & Welcome to Globe.com

You now have unlimited access for the next two weeks.

BostonGlobe.com complimentary digital access has been provided to you, without a subscription, for free starting today and ending in 14 days. After the free trial period, your free BostonGlobe.com digital access will stop immediately unless you sign up for BostonGlobe.com digital subscription. Current print and digital subscribers are not eligible for the free trial.